Removal of oxidized contaminants is an important component of a safe drinking water supply. Biological processes are rapidly gaining acceptance, both domestically and abroad. Treatment systems include biologically-active filters utilizing aerobic processes. However, anaerobic processes can also be used, but have not been commonly employed for purposes of drinking water treatment for reasons relating to the requirement of an electron donor. For instance, methanol, ethanol, and the acetate, which are common organic donors, can cause biological instability, induce taste and odor problems, and may create additional health concerns.
Accordingly, the search for alternate anaerobic water treatments has been an on-going concern in the art. For instance, nitrate, nitrite and other oxidized contaminants are disclosed as removed from drinking water using hydrogen-oxidizing bacteria; that is, biologically with hydrogen as an electron donor. See, U.S. Pat. No. 6,387,262, the entirety of which is incorporated herein by reference.
As described in the aforementioned '262 patent, as an electron donor, hydrogen gas is oxidized by the bacteria with release of electrons for reduction of the contaminant(s). For example, nitrate is reduced in a step wise fashion to innocuous nitrogen gas:NO3−+2H++2e−==NO2−+H2ONO2−+H++e−==NO+OH−NO+H++e−==0.5N2O+0.5H2O0.5N2O+H++e−==0.5N2+0.5H2ONO3−+5H++5e−==0.5N2+2H2O+OH− (overall)
Perchlorate (ClO4−) is an oxidized anion that can originate from a variety of ammonium, potassium, magnesium or sodium salts. Ammonium perchlorate, for example, is a primary ingredient of solid rocket fuel. The short shelf-life of rocket fuel has created an environmental concern given the large volume of perchlorate-containing wastes generated over the years by unused fuel. At least 20 states have confirmed perchlorate contamination, and more sites may be found, as perchlorate has been used or manufactured in up to 40 states. Perchlorate is understood to inhibit thyroid function and is suspect in various other health-related issues. The State of California, recognizing the problem, recently lowered its perchlorate drinking water action level from 18 to 4 μg/L. Even so, a recent toxicological and risk characterization study by the Environmental Protection Agency suggests 1 μg/L as a treatment goal for drinking water.
Perchlorate is not removed by conventional physical-chemical water treatment techniques, and other processes, such as ion exchange, electrodialysis and reverse osmosis are costly and result in a concentrated perchlorate waste stream that still requires disposal. As a result, perchlorate contamination of ground water continues to be an environmental issue.
Perchlorate can be reduced, however, to chloride by perchlorate-reducing bacteria, which use perchlorate as an electron acceptor for growth. Perchlorate-reducing bacteria are readily obtainable in the environment, have a wide range of metabolic capabilities, such as aerobic growth and denitrification, and do not require specialized growth conditions—all attributes suitable for a perchlorate treatment system.
Recent work has shown that bioreactors can reduce perchlorate to below 4 μg/L when the initial concentration is high or when the reactor has been previously operated at high perchlorate concentrations. However, low initial perchlorate concentrations, in the μg/L range, may preclude biomass growth on perchlorate as the sole acceptor electron growth, as predicted by the relationship:
            ⅆ      X              ⅆ      t        =                    q        max            ⁢              S                  S          +          K                    ⁢      YX        -    bX  where S is the rate-limiting substrate concentration [MS/L3], qmax is the maximum specific substrate utilization rate [MX/MS−T], K is the half-maximum-substrate-utilization constant [M/L3], X is the biomass concentration [MX/L3], Y is the biomass true yield [MX/MS], and b [1/T] is the endogenous decay rate. When S is small with respect to K, it can render the positive term on the right side of the equation smaller than the negative term, providing a net decay in biomass for any value of X. Under such conditions, biomass cannot be produced or sustained.
Even so, microbial treatments such as those described in the '262 patent leave several concerns as open issues. For instance, while nitrate/nitrite reduction is discussed, therein, and other oxidized contaminants are mentioned as likewise treatable, concurrent treatment of multiple contaminants remains unaddressed. The '262 patent does not disclose concurrent treatment, and work thereafter appears to indicate full nitrate removal is required for perchlorate reduction to useful levels. As a result, efforts continue in the art to address concurrent treatment of multiple oxidized contaminants, with corresponding movement progress toward a comprehensive treatment methodology.